1. Field of Invention
This invention relates to an improved method of removing mercury and mercury compounds from contaminated soils and industrial waste materials and an apparatus related thereto. More specifically, the invention relates to a system for (a) converting soils and industrial waste products that have been classified as "hazardous" because of their mercury content into materials classified as "non-hazardous" by removing the mercury and (b) recovering and recycling the removed mercury in a manner that minimizes environmental risk and undesired worker exposure.
2. Description of the Prior Art
The disposal of soils and industrial wastes contaminated with mercury or mercury compounds has created major environmental and economic problems for industry. The U.S. Environmental Protection Agency (the "EPA") has classified any industrial waste product having a leachable mercury content of 0.2 parts per million (ppm) or higher (as measured by the standard EPA TCLP Leach Test) as hazardous waste that must be disposed of in a secure hazardous waste land disposal facility. In addition, many state environmental regulatory agencies are requiring that soils contaminated with mercury must be treated to reduce the mercury content to levels equivalent to the background levels that existed in the soils prior to contamination before the that existed in the soils prior to contamination before the treated soils can be disposed of or used as a land fill material.
The relatively low mercury content and large volume of inert materials in contaminated soils and industrial wastes make economic recovery of the contained mercury difficult and the disposal of the contaminated waste extremely costly. As a result, there is a need for a method of removing mercury from soils and industrial wastes that (a) reduces the mercury content of said soils to at least the background level, (b) reduces the mercury content of said industrial wastes to 0.2 ppm or below, and (c) recovers the removed mercury in usable form thereby recovering some economic value and breaking the chain of liability inherent in disposal of mercury-containing wastes classified as hazardous by the EPA.
Mercury contamination of soil is especially acute in the area of metering station sites along natural gas pipelines where, for over 30 years, metallic mercury was routinely discharged onto the ground each time the mercury utilized in flow measuring devices was replaced or failure of said devices occurred. These contaminated sites are characterized by relatively low concentrations of mercury (100 to 2,000 ppm); substantial variations in soil constituents (such as clay minerals, iron/-manganese oxides and hydroxides, gypsum, jarosite, quartz, carbonates, organic and carbonaceous matter, and the like); and the presence of mercury in a wide variety of forms (such as metallic mercury, mercury oxides, mercury sulfides, methyl mercury, and the like). These site characteristics, individually and collectively, prevent efficient, complete and economical removal and recovery of the contained mercury and effective decontaminations of the treated soil. Efficient and economical decontamination of the such sites requires a mercury removal process that is capable of processing soils that very substantially in type and composition, and simultaneously removes the wide variety of forms and compositions of mercury that may exist in the soil. Such a system should reduce the mercury content of the treated soil to the background level of mercury (usually less than 1 ppm) normally present in that particular soil and permits recovery of the removed mercury in reusable form. The equipment should be sufficiently portable to be economically moved from site to site, thereby eliminating the need to transport large volumes of soil to a remote treatment facility in order to remove the mercury therefrom.
Similar problems are encountered when treating mercury-contaminated industrial and incinerator wastes which also vary greatly in mineral form and composition from site to site, contain relatively small amounts of mercury, and are located over a wide geographic area in quantities too small to permit economic recovery of the mercury contained therein.
The removal of mercury from ores and mineral-based materials by thermal means is well known in the art. The mercury contained in such ores and minerals is normally present in the form of metallic mercury that can be vaporized by heating the material to be treated at temperatures above the boiling point of mercury 357.degree. C. (675.degree. F.) or in the form of a crystalline mercuric sulfide (cinnabar) that must be heated to temperatures above 580.degree. C. (1,076.degree. F.) in order to release mercury vapor. See L. H. Duschak and C. N. Schuette, "The Metallurgy of Quicksilver", U.S. Bureau of Mines Bulletin 222, 1925, Pp, 7-11. Vaporization of mercury from such ores and minerals is normally accomplished by heating the ores and minerals in retort furnaces, shaft furnaces, rotary kilns, or multiple hearth furnaces from which the mercury vapor produced is passed into condensers in which it is condensed and collected in metallic form. See L. J. Goldwater, "Mercury--History of Quicksilver, York Press, 1972, Pp. 53-54.
Normally the mercury concentration in the ores or minerals from which mercury is to be extracted is relatively high compared to the mercury concentration in contaminated soils. The mercury concentration in such ores or minerals is usually further increased prior to thermal treatment by such means as crushing, screen, and/or floatation in order to improve the efficiency and economics of mercury removal and recovery and to minimize the amount of mercury vapor discharged from the apparatus. See C. N. Schuette, "Quicksilver", U.S. Bureau of Mines Bulletin 335, 1931, Pp. 63-65.
One known method for removing and recovering mercury from ores by thermal treatment of the ores and condensation of the mercury vapor resulting therefrom is disclosed in U.S. Pat. No. 3,615,363. It involves heating the mercury-containing ore in a non-transportable externally heated, hermetically sealed (relative to the heating source) chamber in which the mercury is vaporized and from which the mercury vapor is removed and transported into a cooled condenser by means of injecting a controllable flow of air into said chamber. The mercury vapor is condensed to form metallic mercury in the condenser by bringing the vapor into contact with the cool condenser walls and the surface of a cool pool of water located at the bottom of the condenser. The condensed mercury collects at the bottom of the water pool under the influence of gravity and is periodically removed therefrom. The gas from which the mercury vapor has been removed is discharged directly from the condenser without additional treatment. Although this and similar methods for removing mercury from ores by thermal means are capable of treating a wide variety of material compositions and recovering mercury from a variety of compounds as well as from metallic mercury contained in the ore, such methods (a) are normally designed to handle feed stocks containing relatively high concentrations of mercury, (b) normally require extensive preparation of the feed material prior to processing, (c) generate large volumes of dust, and (d) normally require large, bulky dust collection equipment to remove the dust from the gaseous effluent prior to discharge of the gases to the atmosphere. Accordingly, the equipment required to thermally treat large quantities of feed stock is normally large, non-transportable, and not generally suitable for removing small quantities of mercury from soils at widely scattered, remote geographic locations.
Methods for removing and recovering mercury from contaminated inorganic industrial wastes and industrial wastes containing organic matter by means of heating the wastes in a vacuum environment are disclosed in U.S. Pat. Nos. 4,087,276 and 4,468,011. U.S. Pat. No. 4,087,276 teaches the removal and recovery of organic industrial wastes containing mercury by heating a mercury-containing inorganic industrial waste material at a temperature of between 200.degree. C. (392.degree.) to 350.degree. C. (662.degree. F.) in a chamber maintained at a negative pressure ranging from 0.06 atmospheres to 0.1 atmospheres in order to vaporize the contained mercury. It also discloses recovering metallic mercury by condensing said mercury vapor in a condenser maintained at a temperature in the range of 20.degree. C. (68.degree. F.) to 50.degree. C. (122.degree. F.) and at a negative pressure approximately equal to that maintained in the chamber.
U.S. Pat. No. 4,468,011 discloses a method of separating mercury from mercury-contaminated battery waste containing plastic organic materials. It involves heating the contaminated waste in a chamber maintained at a negative pressure in order to vaporize the contained mercury, continuously introducing nitrogen gas into the chamber, passing the nitrogen-rich gas containing the generated mercury vapor and organic vapor that exits said chamber through an afterburner in which the organic vapor is completely decomposed and the hydrocarbons resulting from this decomposition are combusted to form CO.sub.2 and water vapor. Subsequently, nitrogen gas containing the CO.sub.2, water vapor, and mercury vapor is passed through successive condensers in which the mercury is condensed and recovered as metallic mercury. The need to maintain a relatively high vacuum and the use of large quantities of inert gas make both of these heretofore patented techniques uneconomical for the recovery of small amounts of mercury from large quantities of contaminated soil and dilute industrial wastes.
Other known means for recovering mercury and usable mercury compounds from mineral feed stocks and industrial wastes include chemical treatment, electrolysis of solutions containing mercury salts, and/or precipitation of mercury ions from mercury-bearing solutions.
U.S Pat. No. 5,013,358 discloses a chemical method for recovering relatively pure elemental mercury from feed stocks prepared from a mineral concentrate or solid inorganic industrial wastes containing small amounts of mercury. It involves preparing an aqueous slurry of the feed stock, solubilizing the mercury contained in the slurry by chlorination of the slurry, treating the chlorinated solution with iron to reduce and precipitate solids containing elemental mercury, and separating the mercury from the precipitated solids.
U.S. Pat. No. 4,879,010 describes electrochemical means for recovery of metallic mercury from Hg.sub.2 Cl.sub.2 by means of dissolving the Hg.sub.2 Cl.sub.2 in an aqueous electrolyte containing HCl. It also discloses recovery of mercury from HgO by dissolving the HgO in an aqueous electrolyte containing glacial acetic acid.
U.S Pat. No. 4,012,297 discloses a chemical process for recovering mercury from solid and liquid industrial wastes generated during the production of chlorine and caustic in mercury amalgam electrolytic cells. The process consists of treating the solid waste with sulfuric acid, solubilizing the waste with an oxidant, treating the resulting mercury-containing solution to form solid mercury sulfide, separating and solubilizing the mercury sulfide, and returning the dissolved mercury to the electrolysis cell for reuse.
Although these known chemical and electrochemical means of removing and recovering mercury from mineral concentrates and liquid and solid industrial wastes can be used to process feed stocks containing low concentrations of mercury, they are not well suited to processing contaminated soils due to the lack of portability of the required apparatus and the wide variation in the chemical and physical make-up of the soils and the mercury species contained therein which cause significant processing difficulties. These difficulties, which result primarily from the unwanted dissolution of non-targeted metals and minerals, include (a) needlessly consuming large amounts of valuable reagents, (b) altering the dissolution kinetics in an uncontrolled manner, (c) contamination of the mercury-containing end product, (d) the need to dispose of and/or treat of large quantities of contaminated liquids, and (e) the possibility of producing another contaminated solid waste for disposal or treatment.
Other known techniques for treating mercury-contaminated soils and industrial wastes prior to disposal involve converting the contained mercury to relatively insoluble species and encapsulating the insoluble species such that the leachable mercury contained in the treated soil or waste is less than the limit established by the EPA in defining "hazardous waste". Various methods of stabilizing and encapsulating mercury compounds in treated soils and industrial wastes are disclosed in U.S. Pat. Nos. 4,853,208, 4,844,815, 4,921,538, 4,977,837, and 5,037,286.
U.S. Pat. No. 4,853,208 discloses a method of physically and chemically encapsulating mercury species contained in mercury-containing industrial wastes by treating the wastes with an additive (i.e., mercaptan), an alkali metal silicate, and a setting agent such as cement such that, upon setting, the treated waste is rendered non-polluting.
U.S. Pat. No. 4,844,815 discloses a method of reducing the leachability of mercury contained in industrial wastes that involves mixing the waste with elemental sulfur, a strong caustic, and cement kiln dust and allowing the mixture to cure prior to disposal.
U.S. Pat. No. 4,921,538 discloses a method for recycling mercury-containing soils and industrial wastes that comprises combining the soil or industrial waste with a mixture of inorganic materials such as calcium oxide, silica, aluminum oxide, iron oxide, and magnesium oxide in proportions that result in a final mixture having a composition suitable for the production of Portland Cement, preparing a slurry of said final mixture, and heating the slurry in a cement kiln to a temperature sufficient to form a cement clinker that passes the standard EPA TCLP Leach Test pertaining to mercury leachability.
U.S. Pat. No. 4,354,942 discloses a method of preventing the leaching of soluble mercury species from deposits of mercury-containing materials by stabilizing the soluble mercury species by treating the deposits in situ with an inorganic sulfur compound such as a metallic sulfate or thiosulfate.
U.S. Pat. No. 4,977,837 discloses a process and apparatus for reducing the leachability of mercury contained in incinerator ash by mixing the ash with a glass material and fusing the mixture such that the fly ash becomes vitrified by combining with the glass. Another method for limiting the leachability of mercury contained in incinerator wastes, disclosed in U.S. Pat. No. 5,037,286, is based upon mixing the wastes with Portland Cement, aggregating the mixture into balls, and creating a cement coating on the surface of said balls.
Although the aforesaid prior art methods and techniques of stabilizing and encapsulating mercury species contained in soils and industrial wastes are capable of significantly reducing the leachability of mercury from said soils and wastes, they greatly increase the volume of material requiring disposal and do not reduce the mercury content of the treated product, thereby extending the chain of environmental liability inherent in the disposal or re-use of the treated material.
None of the heretofore discussed prior art treatment methods or apparatus are adaptable to the economical, on-site reduction of the leachable mercury content in mercury-contaminated soils and industrial wastes, conversion of materials classified as "hazardous waste" to materials classified as "non-hazardous waste", and recovery of the removed mercury in usable form thereby ending the environmental liability related to the treated soil or waste.
There remains, therefore, a need for a practical method of (a) economically removing mercury from contaminated soils and industrial wastes that vary greatly in composition, physical form, and the species of mercury contained therein; (b) on-site conversion of mercury-containing soils and industrial wastes classified as "hazardous" to conversion products classified as "non-hazardous" by removal of the mercury therefrom; (c) recovering the removed mercury in usable form; and (d) thermally treating mercury-bearing soils and industrial wastes in a manner that permits control of the mercury, sulfur, and dust contents in the gaseous process effluent to a degree that the effluent is considered harmless to the environment and to persons practicing the method. Such a system incorporating these improvements would substantially reduce the cost of treating mercury contaminated soils and wastes and greatly reduce the environmental dangers inherent therein.